1. Field of the Invention
The application relates to a method utilized in a wireless communication system, and more particularly, to a method of handling uplink time alignment in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
According to structure of the LTE system, time alignment (TA) functionality allows a user equipment (UE) with a component carrier to be synchronized with a serving base station on uplink timing for preventing signals transmitted from the UE from colliding with those sent from other UEs under the coverage of the base station. In the TA functionality, the UE has to maintain a time alignment timer whose running state indicates that uplink transmission is still synchronized. The network controls the TA functionality of the UE with a timing advance command. In detail, the network uses a MAC Protocol Data Units (MAC PDU) to transmit the timing advance command to the UE. A MAC PDU consists of a MAC header, zero or more MAC Service Data Units (MAC SDU), zero, or more MAC control elements, and optionally padding. A MAC PDU header consists of one or more MAC PDU subheaders, and each subheader corresponds to either a MAC SDU, a MAC control element or padding. A MAC PDU subheader consists of the six header fields R/R/E/LCID/F/L but for the last subheader in the MAC PDU and for fixed sized MAC control elements. The last subheader in the MAC PDU and subheaders for fixed sized MAC control elements consist solely of the four header fields R/R/E/LCID. A MAC PDU subheader corresponding to padding consists of the four header fields R/R/E/LCID.
Moreover, MAC PDU subheaders have the same order as the corresponding MAC SDUs, MAC control elements and padding. In addition, the MAC control elements are always placed before any MAC SDU. Padding occurs at the end of the MAC PDU. Note that, a MAC control element carrying the timing advance command is called a timing advance command MAC control element, and is identified by a MAC PDU subheader with a LCID.
Toward advanced high-speed wireless communication system, such as transmitting data in a higher peak data rate, LTE-Advanced system is standardized by the 3rd Generation Partnership Project (3GPP) as an enhancement of LTE system. LTE-Advanced system targets faster switching between power states, improves performance at the cell edge, and includes subjects, such as bandwidth extension, coordinated multipoint transmission/reception (COMP), uplink multiple input multiple output (MIMO), etc.
For bandwidth extension, carrier aggregation is introduced to the LTE-Advanced system for extension to wider bandwidth, where two or more component carriers are aggregated, for supporting wider transmission bandwidths (for example up to 100 MHz) and for spectrum aggregation. According to carrier aggregation capability, multiple component carriers are aggregated into overall wider bandwidth, where the UE can establish multiple links corresponding to the multiple component carriers for simultaneously receiving and transmitting. In carrier aggregation, the UE only has one RRC connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the NAS mobility information, and at RRC connection re-establishment/handover, one serving cell provides the security input. This cell is referred to as a primary cell (PCell). In the downlink, the component carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC). In addition, cells other than the PCell are named secondary cell (SCell).
Note that, the PCell (i.e. the UL and DL PCC) is always activated, whereas the SCell may be activated or deactivated according to specific conditions (e.g. an amount of data for transmission). The UE shall not monitor the physical downlink control channel (PDCCH) of a deactivated SCell and shall not receive any downlink assignments or uplink grants associated to a deactivated SCell. The UE shall not transmit on UL-SCH on a deactivated SCell. In addition, the network activates and deactivates the SCell by sending the Activation/Deactivation command.
According to the prior art, it is possible to configure a UE of a PCell and one SCell or more SCells. Therefore, multiple timing advances are needed for PCell and SCell or more SCells in different bands. To get a multiple timing advance for a configured SCell, the UE performs a random access procedure by transmitting a preamble on the configured SCell. However, the LTE-Advanced system does not clearly specify from where (e.g. PCell or which SCell) the UE should receive a random access response including a timing advance command. Furthermore, the LTE-Advanced system does not clearly specify how to handle the configured SCell when the UE fails to complete the random access procedure.
A conventional MAC control element contains one timing advance command for updating uplink timing of a UE in RRC connected mode. When the UE receives the conventional MAC control element, the UE does not know which cell (e.g. PCell or SCell) the timing advance command should be applied. Besides, if a network wants to update uplink timing for the uplink component carriers (i.e. 5 component carriers), the network has to send 5 MAC control elements in 5 MAC PDUs, which is not efficient.